Workers have built more than 5,000 linear of booms since January to keep floating logs and debris out of the main channel of the Tuolumne River Arm of Don Pedro Reservoir. (Guy McCarthy / Union Democrat) - union democrat

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Workers have built more than 5,000 linear of booms since January to keep floating logs and debris out of the main channel of the Tuolumne River Arm of Don Pedro Reservoir. (Guy McCarthy / Union Democrat)6265571

Chief Ranger Denton Sturdivan of Don Pedro Recreation Agency looks from his wheelhouse to co-workers on a tug and a barge Thursday in the Tuolumne River Arm of Don Pedro Reservoir. (Guy McCarthy / Union Democrat) - union democrat

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Chief Ranger Denton Sturdivan of Don Pedro Recreation Agency looks from his wheelhouse to co-workers on a tug and a barge Thursday in the Tuolumne River Arm of Don Pedro Reservoir. (Guy McCarthy / Union Democrat)6265573

Jeremy Scharlie and Seth Powell kneeled on a work barge in the Tuolumne River arm of Don Pedro Reservoir Thursday and swung metal-head sledgehammers to drive steel spikes into a floating log.

The spikes had metal eyelets for braided steel wire, which they used to secure the log and add it to more than 5,000 linear feet of booms they’ve built since January to harness all the dead wood that’s washed down into the state’s sixth-largest capacity reservoir.

Tons of limbs, logs, stumps and other wood debris are floating just downstream from Wards Ferry Bridge. Some of the wood is burned black, perhaps from the 2013 Rim Fire. Much of the floating debris is already bleached gray, white and silver-white by the sun.

Don Pedro Recreation Area staff say one of the wettest winters on record has washed down four times the amount of wood that normally comes into the reservoir each spring.

Chief Ranger Denton Sturdivan kept an eye on the workers from the wheelhouse on his 21-foot patrol boat.

1,500 square miles of forests

“We have about 40 acres of debris up here this year,” Sturdivan said. “In a normal year it’s about 10 acres.”

The water in Don Pedro comes from a Central Sierra watershed of more than 1,500 square miles that includes much of the drought-stricken, beetle-infested Stanislaus National Forest, so it’s no surprise to see a lot of dead wood floating in the reservoir.

Dealing with logjams and debris fields is a necessary maintenance ritual that happens each and every spring at Don Pedro, said Calvin Curtin, a spokesman for Don Pedro Recreation Agency and Turlock Irrigation District.

“In drought years, when there’s less water, there’s less logs and debris carried down into the river and into the reservoir,” Curtin said. “In wet years like this one, a lot more water and runoff and snowmelt comes down, and so do a lot more trees.”

Logjams

Logjams and floating debris are concerns at Don Pedro for three primary reasons.

It’s a life-safety issue for boaters and skiers, Curtin said. Bass boats can reach 50 to 60 miles per hour on open stretches of the man-made lake. Striking a submerged, 4-foot-diameter log at those speeds can damage props, motors and hulls.

Clusters of logs are impassable for most boats. That’s why Jigour drives a tug and uses it to tow Sturdivan’s patrol boat through the thickest debris fields.

Second, Turlock Irrigation District staff don’t want to see sections of former 150-foot-tall trees get caught up in their control structures, including the dam, powerhouse, controlled spillway and uncontrolled spillway.

Third, the staff at Don Pedro Recreation Agency try to keep the Tuolumne River Arm clear of debris for rafters and other river boaters who come down the Tuolumne and take out at Wards Ferry Bridge.

Turlock Irrigation District staff say that floating woody debris, if left uncontained, can move upstream and completely block the river channel at high reservoir levels when inflow slows down.

Fixing the problem

As of Thursday, Don Pedro held more than 1.68 million acre-feet, 83 percent of capacity. The surface elevation was 801.48 feet above sea level.

River currents, sudden changes in inflows after storms and during snowmelt, and accelerated releases to draw the reservoir down can all cause water movement on Don Pedro’s surface. Water movement can move all those floating logs and other debris around, upstream and downstream.

Back in January, the logjam reached 2 miles below Wards Ferry Bridge. By this week, Scharlie, Powell and Jigour had corralled most of the floating debris and the last loose remnants of free-moving detritus was about a half-mile downstream from Wards Ferry.

Their strategy includes building log booms to fence in all the floating debris, then lash these bundles of limbs and logs to the shore. Each year, Sturdivan said, the reservoir gets drawn down as farmers and other users take water for irrigation. As the water level drops, it leaves wired clumps of wood debris beached on the steep canyon sides of the Tuolumne River Arm.

When fall arrives and open burning is allowed, Don Pedro Recreation Agency staff will return by boat and set the dried-out debris ablaze, burning as much as they can.

‘Boat with caution’

For now, what they call their floating debris containment area is about four miles upstream from Moccasin Point and it’s expected to remain closed to boating until mid-May.

There’s a downstream debris barrier boom just above Rough n Ready Creek that will be closed during significant storm events and during Hetch Hetchy Water & Power releases if high inflow into the reservoir is expected.

As reservoir levels rise again during snowmelt in May and June, beached debris along the shoreline is likely to refloat and become navigational hazards.

Meanwhile, aside from the Tuolumne River Arm, there is plenty of room for boaters to move around on Don Pedro. The reservoir’s surface area as of Thursday was about 11,235 acres, more than 17.5 square miles. Though Don Pedro’s level has dropped since late February, it is still fuller than it’s been in the past five years.